One type of an infrared detector consists of a microbolometer responsive to the heat energy of infrared radiation. These microbolometers are typically grouped on a single substrate into an array of microbolometers. Microbolometer construction includes a sensor portion or pixel bridge, readout circuitry, and interconnections. In certain microbolometers, the pixel bridge is formed from an oxide of vanadium (VOx), which has a high temperature coefficient of resistance (TCR) making it an excellent IR sensing material.
A conductive path is needed to provide the electrical connection between the vanadium oxide (VOx) of the pixel bridge and the underlying readout integrated circuit (ROIC). Part of this connection is a metal trace leading from the pixel bridge to the metal support structure. This metal trace is sometimes referred to as the leg. Ideally, the VOx deposited for the bridge could also act as the material for the electrical interconnect. However, the VOx used in the pixel bridge cannot be used as the electrical trace because it is not metallic; i.e., its electrical resistance is too high.
The use of a metal or metal alloy such as NiCr as the electrical interconnect requires significant additional processing such as extra insulating dielectric, metal deposition, and multiple associated added photolithography, etching and cleaning steps. The added steps occur whether the metal interconnect process is done before or after VOx deposition.
Each step arising from the need for the added metallization negatively affects the cost and schedule of building microbolometers, and increases the chance of lower yield due to loss of pixels and die from the multiple processing steps. Such steps can also produce more particulates with a further impact on yield. In addition, as state of the art pushes pixels to become ever smaller and the films thinner, these processing steps become more difficult to scale for smaller geometries. For example, continuous step coverage of the thin metal over the dielectric into the detector contact area becomes a more significant issue if the topology of the pixel is not minimized.
An alternative leg approach involves back-sputtering the VOx to change its electrical properties. An example includes U.S. Pat. No. 6,144,285 to Higashi incorporated herein by reference. Here, exposed microbolometer legs are subjected to argon gas back-sputtering to decrease the resistance of the VOx by converting it to another form of vanadium oxide. Back-sputtering, however, erodes the protective dielectric in the bridge area, particularly the edges, which can lead to electrical shorts along the perimeter of the bridge. Increasing the back-sputtering energy for still lower resistance results in higher erosion of the protective oxide or dielectric. In addition, back-sputtering is primarily a surface phenomenon and has poor control over the conversion depth of the VOx resulting in less predictable leg resistances. Back-sputtering with argon is essentially a cleaning process and does not provide the desired control. Finally, back-sputtering does not address converting the VOx at the interface of the leg contact to the metal support structure. Excessive contact resistance in this area degrades device performance.
A need, therefore, exists for fabrication methods using highly controllable, precise, reproducible means and supporting fewer steps leading to better scaling, lower cost, faster production, and greater yield.